The present invention relates to switching regulator circuits. More particularly, the present invention relates to circuits and methods for providing slope compensation signals for voltage regulators based on input and output voltages.
The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a voltage source which may be poorly-specified or fluctuating. Two types of regulators are commonly used to provide this function, a linear regulator and a switching regulator. In a typical linear regulator, the output voltage is regulated by controlling the flow of current through a pass element from the voltage source to the load.
In switching voltage regulators, however, the flow of current from the voltage source to the load is not steady, but rather in the form of discrete current pulses. To create the discrete current pulses, switching regulators usually employ a switch (such as a power transistor) that is coupled either in series or parallel with the load. The current pulses are then converted into a steady load current with an inductive storage element.
By controlling the duty cycle of this switch (i.e., the percentage of time that the switch is ON relative to the total period of the switching cycle), the switching voltage regulator can regulate the load voltage. In current-mode switching voltage regulators (i.e., a switching regulator that is controlled by a current-derived signal in the regulator) there is an. inherent instability when the duty cycle exceeds 50% (i.e., when the switch is ON for more than 50% of a given switching period). Stability is often maintained in such current-mode switching regulators by adjusting the current-derived signal used to control the regulator with a slope compensation signal.
One method of producing such slope compensation signals is to use a portion of a ramp signal as the compensation signal. The ramp signal may be, for example, an oscillator signal that is used to generate a clock signal that controls the switching of the regulator. The slope compensation signal can be applied by either adding the ramp signal to the current-derived signal, or by subtracting it from a control signal.
An example of a typical prior art circuit 10 that provides slope compensation for a switching voltage regulator is shown in FIG. 1. The circuit of FIG. 1 operates as follows. Oscillator circuit 30 provides a ramp signal such as a sawtooth waveform to the base of transistor 20. As the sawtooth waveform ramps up, transistor 20 begins to conduct, and current flows from voltage source 28 to resistor 22 creating a voltage at node 23, which is applied to the non-inverting input 34 of amplifier 32. Generally speaking, as the sawtooth waveform increases in magnitude, so does the voltage at node 23 and vice versa. This signal is generally known as the slope compensation signal. Usually, the sawtooth waveform produced by oscillator 30 is substantially in-phase with a clock signal that is used to coordinate the switching of a power transistor (not shown) within the voltage regulator. This is done to ensure that slope compensation is provided at the proper time relative to the duty cycle of the power transistor (e.g., when the duty cycle exceeds a predetermined value). The maximum amount of slope compensation is provided when the sawtooth waveform reaches its peak, and conversely, the minimum amount of slope compensation is provided (if any) when the sawtooth waveform is at its minimum.
The current provided by the voltage regulator is monitored by sensing the output current present in a storage inductor (not shown) located in the output stage of the voltage regulator. This current is measured in FIG. 1 by passing a signal indicative of the output current through sensing resistor 26. This creates a voltage at node 25 that indicates the amount of current the voltage regulator is providing. This voltage is sensed at error amplifier 32 by measuring the voltage drop between non-inverting terminal 34 and inverting terminal 36 (i.e., across a current sense resistor 26). The voltage regulator compares the output of current sense amplifier 32 to a preset threshold value to determine when to open and close a power switch that provides current to the load.
Slope compensation is provided in FIG. 1 by adding the voltage present at node 25 with the slope compensation voltage provided at node 23. With no slope compensation provided, the voltage at non-inverting terminal 34 is approximately equal to the voltage at node 25. When slope compensation is provided, however, and the sawtooth waveform progresses toward its peak, the voltage at node 23 rises, which consequently increases the voltage at non-inverting terminal 34. The voltage regulator interprets this as an increase in the rate of current rise in the output inductor. This causes the perceived rate of current rise in the inductor to be greater than the rate of current fall, which allows the voltage regulator to operate at duty cycles greater than 50% without becoming unstable.
One shortcoming of this technique is that it fails to produce slope compensation with respect to the input voltage provided to the regulator. This is a significant deficiency because the value of the input voltage directly effects the duty cycle of the regulator. For example, as input voltage decreases, the duty cycle must increase to maintain output voltage. Thus, slope compensation must increase accordingly to ensure regulator stability.
In the past, circuit designers have accounted for this problem by providing slope compensation based on xe2x80x9cworst-casexe2x80x9d input voltage conditions. This, however, often results in the production of excessive amounts of slope compensation, which is generally undesirable, because it can significantly reduce the response time of the regulator.
It would therefore be desirable to provide a slope compensation circuit that provides slope compensation to a switching voltage regulator as a function of input voltage.
It would also be desirable to provide a slope compensation circuit that provides optimum amounts slope compensation based on the amount needed to ensure regulator stability.
It is therefore an object of the present invention to provide a slope compensation circuit that provides slope compensation to a switching voltage regulator as a function of input voltage.
It is another object of the present invention to provide a slope compensation circuit that provides optimum amounts slope compensation based on the amount needed to ensure regulator stability.
These and other objects of the present invention are accomplished by providing a slope compensation circuit that provides slope compensation as a function of both input voltage and output voltage. This allows the slope compensation circuit to provide the optimum amount of slope compensation so that the response time of the voltage regulator is improved and the current limit effects of slope compensation are minimized.
The slope compensation circuit includes a control circuit, a feedback circuit, and a slope signal generator circuit. The feedback circuit produces a feedback signal which is a function of both input voltage and output voltage. The control circuit generates a control signal based on the feedback signal that varies the impedance of circuit elements within it to establish the slope of current that can be conducted by the slope signal generator circuit. This allows the slope signal generator circuit to produce slope compensation signals that are specifically tailored to the stability requirements of the regulator in view of the input and output voltages.